1. Field of the Invention
The present invention relates to a device for amplification of light pulses, having an optical stretcher, in which the light pulses of a pulsed laser light source are temporally stretched, and having an optically pumped amplifier fiber, in which the light pulses are amplified and temporally compressed.
2. The Prior Art
Laser systems that are able to produce femtosecond light pulses are increasingly being used in basic physical research and also in other areas of research. Using such laser systems, it is possible to observe rapid physical, chemical, and biological processes essentially in “real time.” Commercial areas of use for laser systems that produce femtosecond light pulses exist in the fields of materials examination and processing, in the field of medicine, as well as in the so-called “life science” field. Concrete applications that can be mentioned as examples are multi-photon microscopy as well as optical coherence tomography.
Furthermore, only a few years ago, the use of phase-stabilized laser systems for producing femtosecond light pulses as highly precise frequency references became known. This development makes it possible, for example, to measure optical transitions with great accuracy. By means of a direct link between the optical spectrum range and the most precise clocks, at this time, in the range of microwave and radio frequencies, such systems replace complicated and expensive frequency division chains. In the field of optical frequency metrology, as well, there are interesting areas of application for laser systems that produce femtosecond light pulses.
Passive-mode-coupled glass-fiber laser systems that yield light pulses having pulse durations of ≦100 femtoseconds have been known for several years and have become established in optical laboratories. Fiber-based laser systems have the advantage of a particularly great cost efficiency, and a very low maintenance and adjustment expense. In addition, as compared with free-beam laser systems, extremely compact units can be achieved. In the case of the fiber-based systems, optical fibers doped with rare earths are generally used as the laser-active medium. When using fibers doped with erbium, an emission wavelength in the range of 1.55 μm is obtained. It is preferable if laser systems that emit in this wavelength range are compatible with a large number of optical components that are used in the sector of telecommunications technology. However, it must be stated that fiber-based laser systems have disadvantages, as compared with the known titanium-sapphire free-beam laser systems, with regard to the pulse duration and pulse energy that can be achieved. In the case of pulse durations of ≦100 femtoseconds, at this time it is possible to achieve pulse energies of only about 100 picojoules, using purely fiber-based laser systems. The comparatively low light intensity that results, however, is insufficient for many applications, particularly in the field of non-linear optics.
It has therefore been shown that in order to achieve pulse energies of a nanojoule and more, it is necessary to further amplify the light pulses produced by known fiber-based laser systems. A device that is suitable for this purpose is previously known from U.S. Pat. No. 5,880,877, for example. In the previously known system, an optically pumped amplifier fiber is used, in which the light pulses produced by a pulsed laser light source are amplified. In this connection, the amplifier fiber has a negative group velocity dispersion in the corresponding wavelength range (i.e. the fiber is operated in the anomalous-dispersion regime). The solitonic optical effects that are promoted thereby are utilized, in the previously known system, in order to generate light pulses with an extremely short pulse duration, in the range of 30 femtoseconds. It is problematic that although the corresponding solitones have a short and intense peak, pulse wings having a temporally wide spread are formed, and that a significant part of the total pulse energy goes to these pulse wings. In order to eliminate this undesirable temporal pulse shape, a frequency doubler is used in the previously known system, which is a medium having non-linear optical properties. Because of their comparatively low intensity, the aforementioned pulse wings do not contribute to the frequency-doubled light pulse, so that in total, a correction of the temporal pulse shape is achieved. A significant disadvantage of the previously known system is that, for one thing, a significant energy loss is unavoidably connected with the frequency doubling. Furthermore, the wavelength of the generated light pulses leave the technologically interesting wavelength range around 1.55 μm, because of the frequency doubling.